Color-plus-clear composite coating systems involving the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear topcoat to the base coat have become conventional as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image, due in large part to the clear coat.
Coating systems containing hydroxyl functional polymers cured with aminoplasts are also well known and provide many excellent coating properties. They are inexpensive, durable, and attractive. However, it is widely recognized that such coatings, particularly clear coats, have poor resistance to etching by acid.
It has been found that acid etch resistance of such coatings can be improved by using high molecular weight, hydroxyl functional polymers having relatively high glass transition temperatures in combination with relatively low levels of aminoplast. However, such coating compositions have viscosities that are too high for conventional spraying techniques. The viscosity may be lowered by dilution with a solvent or diluent, but dilution adds volatile components which must be disposed of or recovered, adding to the cost of the coating system. The diluted coating composition also typically suffers from appearance problems upon application such as drips, sags, solvent popping, and the like.
It is an object of the present invention to provide a method of applying a color-plus-clear composite coating composition to a substrate, wherein the clear coating component of the composition is based on hydroxyl functional polymers and aminoplast curing agents, and the cured composition demonstrates a high level of acid etch resistance and excellent appearance properties without requiring high levels of solvents in the formulations.